An electroluminescent (EL) device is a self-light-emitting device which has advantages in that it provides a wider viewing angle, a greater contrast ratio, and a faster response time. An organic EL device was first developed by Eastman Kodak, by using small aromatic diamine molecules and aluminum complexes as materials to form a light-emitting layer [Appl. Phys. Lett. 51, 913, 1987].
In an organic electroluminescent device (OLED), electricity is applied to an organic light-emitting material which converts electric energy to light. Generally, OLED has a structure comprising an anode, a cathode, and an organic layer disposed between the two electrodes. The organic layer of OLED may comprise a hole injection layer, a hole transport layer, an electron blocking layer, a light-emitting layer (comprising a host and dopant materials), an electron buffering layer, a hole blocking layer, an electron transport layer, an electron injection layer, etc. A material for preparing the organic layer can be classified according to its function, as a hole injection material, a hole transport material, an electron blocking material, a light-emitting material, an electron buffering material, a hole blocking material, an electron transport material, an electron injection material, etc. Holes and electrons are injected from an anode and a cathode, respectively, to the light-emitting layer by applying electricity to OLED; excitons having high energy are formed by recombinations of the holes and the electrons, which make organic light-emitting compounds be in an excited state, and the excited state of the light-emitting compound return to a ground state, which converts energy into light, and results in light-emission.
The most important factor determining luminous efficiency in OLED is a light-emitting material. The light-emitting material needs to have high quantum efficiency, high electron mobility, and high hole mobility. Furthermore, the light-emitting layer formed by the light-emitting material needs to be uniform and stable. According to colors visualized by light-emission, the light-emitting material can be classified as a blue-, green-, or red-emitting material, and a yellow- or orange-emitting material can be additionally included therein. Furthermore, the light-emitting material can be classified according to its function, as a host material and a dopant material. Recently, the development of OLED providing high efficiency and long lifespan is urgent. In particular, considering EL requirements for middle or large-sized OLED panels, materials showing better performances than conventional ones must be urgently developed. In order to achieve the development, a host material, which plays a role as a solvent in a solid state and an energy carrier, should have high purity, and an appropriate molecular weight for being deposited in vacuum. In addition, a host material should have high glass transition temperature and high thermal decomposition temperature to ensure thermal stability; high electrochemical stability to have long lifespan; ease of preparation for amorphous thin film; and good adhesion to materials of adjacent layers. Furthermore, a host material should not move to an adjacent layer.
The light-emitting material can be prepared by combining a host with a dopant to improve color purity, luminous efficiency, and stability. Generally, a device showing good EL performances comprises a light-emitting layer prepared by doping a dopant to a host. The host material greatly influences the efficiency and lifespan of the EL device when using a host/dopant system, and thus its selection is important.
Meanwhile, Korean Patent No. 10-1405725 discloses an organic light-emitting device comprising an amine derivative substituted with an aryl group or a heteroaryl group as a host of a light-emitting layer, and further discloses some hosts capable of optionally being used as a host besides the amine derivative. However, the above publication does not suggest improving a luminous efficiency by combining hosts having specific structures compared to using a sole host compound.
In this regard, the present inventors have found that an improved efficiency could be achieved by using a plurality of host materials comprising an amine-carbazole derivative substituted with naphthyl-phenyl and aryl, and a di-C-benzocarbazole derivative as a host, compared to using a conventional sole host material.